1. Field of the Invention
The invention relates to apparatus for floatingly suspending and guiding a running web of indeterminate length through an elongated dryer. More particularly the invention relates to a control system for controlling the exhaust flow rate of air from the dryer and in which the exhaust flow rate requirement is directly related to the press speed.
2. Background Information
In high velocity web dryers, safe dryer design requires that the exhaust system must be capable of handling the worst possible condition for solvent laden air. This "worst case" condition would occur at a web speed equal to the printing press maximum web speed while laying down maximum solvent onto the paper. Maximum web speed would draw the most solvent laden paper through the dryer while the heaviest lay down has the highest concentration of solvent. The exhaust system's purpose is to remove this solvent from the dryer as it is evaporated from the web. This exhaust flow out of the dryer must be sufficient to remove the solvent laden air under the worst condition of operation.
The present invention is utilized with dryer apparatus of the type shown in U.S. Pat. No. 4,833,794 issued May 30, 1989, to Stibbe, et al, U.S. Pat. No. 4,787,547 issued Nov. 29, 1988 to Hella, et al, or in U.S. Pat. No. 4,116,620 issued Sep. 26, 1978 to Stibbe. These high velocity web dryers all utilize air bars for floatingly suspending the running web as it moves through an elongated housing and have exhaust systems for exhausting the solvent laden air from the housing. Prior art devices use various means to determine the correct exhaust flow rate of the air. Many of these systems use energy which is based on the air flow through the system and not particularly on the solvent content of the air.
In actual practice, presses rarely reach their maximum speeds, generally running at 60% to 80% of maximum speed. This reduction in speed directly reflects the concentration of solvent in the air.
A reduction of exhaust rates has several benefits to the printer. Since the exhaust system is removing heated air from the dryer, less energy is required to maintain a set temperature because less heat would be removed.
When air is exhausted from the dryer, other air must take its place. This "make-up" air is drawn from the room area surrounding the dryer. This room air, in turn, must also be replaced, and usually this replacement air is drawn from outside the plant, meaning it must be heated or cooled to maintain a comfortable working environment. Reducing this air inflow to the plant will reduce the printers heating/cooling cost.
Furthermore, printers are generally required to include pollution control devices on their exhaust systems. A majority of systems currently on the market use energy based on air flow through the system, not just the solvent content of the air. By reducing the air flow, the printer would save additional expense.